Like the mainline Pokémon games, Pokémon GO has various stats for each Pokémon. Simplified over the main games, each Pokémon has got three stats: Hit Points, Attack and Defense. These stats are used to calculate the Health of the Pokémon as well as how much damage they can inflict and receive in the Gym Battles. The Attack and Defense stats are hidden while Health is calculated and visible easily

While these stats are simplified versions of the main line games, they are actually calculated based on the base stats of the Pokémon from the main games: GO HP = 2 * HP Stat
GO Attack = (Attack * Special Attack)^0.5 + Speed^0.5
GO Defense = (Defense * Special Defense)^0.5 + Speed^0.5
Then, when calculated, each Pokémon has got an Individual Value from 0 to 15 which creates variation in different Pokémon of the same Pokémon species. The higher the IV, the stronger the Pokémon. You have a 1 in 4,096 chance of having a Pokémon with perfect IVs in Pokémon GO.